Ethylenically unsaturated organic compounds, capable of addition polymerization, such as acrylic acid, methacrylic acid, methacrylic esters, acrylate esters and the like are widely used in the production of homopolymers and copolymers. These homopolymers and copolymers, produced readily through the polymerization of the available double bonds of the organic compounds, are widely used in paints, coatings, lacquers and the like. The olefinic activity of the ethylenically unsaturated organic compounds, makes the polymerized products highly useful for many purposes. On the other hand, the olefinic activity poses a problem of unintentional or premature polymerization promoted by light and heat. This tendency is exhibited on storage, shipping or even during purification of these polymerizable unsaturated compounds where distillation at higher temperatures are involved.
It is the practice to employ stabilizers to inhibit the setting of polymerizable ethylenically unsaturated organic compounds. For example, in the stabilization of acrylic acid and other unsaturated compounds, inhibitors commonly used are phenothiazine, methylene blue, hydroquinone in the presence of oxygen, N,N'-diphenyl-p-phenylenediamine, the monomethyl ether of hydroquinone and the like. The amount of inhibitor customarily used is sufficient to prevent hazardous polymerization even after prolonged storage at normal temperatures. For acrylic acids the amounts commonly used for storage include: phenothiazine, about 200 parts per million; methylene blue 0.5-1%; hydroquinone, 0.1%; N,N'-diphenyl-p-phenylenediamine, 0.05%; and the monomethyl ether of hydroquinone, 0.05-0.1%. Removal or reduction in amounts of these stabilizers from the ethylenically unsaturated organic monomer product, prior to the polymerization reaction, is generally carried out by distillation in instances at reduced pressures. Even at low pressure, distillation of these monomers is difficult because polymer forms readily. During the distillation the liquid phase in the distillation unit should contain a liquid phase nonvolatile inhibitor and the column and condenser should contain a volatile inhibitor such as nitric oxide to prevent the undesired or premature polymerization of the ethylenically unsaturated organic monomer product.
It is not generally known to use a metal additive in combination with phenolic inhibitors to prevent undesirable polymerization of ethylenically unsaturated organic compounds. In Chemical Abstracts, Vol. 74, 1971, 112642V, a Japanese patent (No. 70-35,285) filed June 1, 1967, assigned to Nippon Kayaku Co., Ltd. describes the mixture of chromium acetate with hydroquinone as a satisfactory inhibitor for acrylic and methacrylic acid. In Japanese patent publication No. 51-98211, filed Feb. 19, 1975, assigned to Sumitomo Chemical Co., Ltd., the combination of manganese salt and phenolic type inhibitors provide satisfactory stabilization of acrylic acid. The use of cerium is not known as an inhibitor alone or in combination with other inhibitors as a stabilizer for the polymerizable ethylenically unsaturated organic compounds.